edit the template_fboxes.php file and change the line that says $perline=3 to 4
Then create css that looks like this
#container .fboxes .fbox {width:230px;}
(not sure on the exact width change)
You will also have to mess with the background image which controls the lines, but we actually can create css for this too.
Using firebug will help a lot. Let me know if you further assistance I just don't know how much info to give you all at once

you should change the width to 239px or add padding of some sort to fix the alignment.
To add a fourth feature box just click the "add feature box" (top right corner) button in the theme options > feature setup area.